If you want to see a hideous, hideous picture of one, take a look at Wikipedia. The term is a corruption for 'jeune d'Anvers', or 'young girl from Antwerp', but I can't figure out why they would call this that, unless Antwerp girls were TERRIFYINGLY HIDEOUS.
a number of peculiar creatures were observed in Paris by Hieronimus Cardanus (a sixteenth century Italian mathematician and physician) and he wrote describing them: "two-footed creatures with very small wings, which one could scarcely deem capable of flight, with a small head . . . like a serpent, of a bright colour, and without any feather or hair." These creatures were about the size of small rabbits. They were probably Jenny Hanivers - fakes made from composites of the bodies of actual beasts, for example the fake "mermaids" created from monkeys sewn onto fish-tails. Jenny Hanivers were immensely popular during the sixteenth and seventeenth centuries, and Jenny Haniver dragons were probably made from sea skates or rays.